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  • Hum problems with Gulbransen Model D



    Hi everyone. I have still no been able to figure this out and need a little help. </P>


    I have had constant hum-related problems with my Gulbrasnen Model D. I had all the filter capacitors replaced not once, but twice, by two different people (there are two power supplies - one for console power, the other for amp power - this is all solid state). I have tried putting a scope on here several times and I was not reading much ripple current at all; on one of the supplies there was a tiny bit of AC and I just wired in a new cap in parallel and that reduced the hum. I can get the hum quieter but not gone. Then, days later, there will be hum again. It sounds more like 60Hz than 120Hz. Here are some weird things and maybe you can make heads or tails out of it:</P>


    1. Since the second rebuild I noticed that when I turn the organ off, a louder "buzzing" sound is produced, then slowly fades away. It sounds like something is being discharged. The only thing that made me curious was that two 1500uF caps in the amp PS unit were replaced with 5000uF caps, but I was told that was OK. </P>


    2. The hum does not get louder when I turn up the expression pedal. Strangely enough, the hum is actually more noticable when the expression pedal is all the way down (quietest).</P>


    3. My organ has a switched AC receptacle. If I plug certain things into this receptacle (like my rhythm box), the hum gets louder, even if I do not have therhythmbox powered on. But, then I tried plugging a simple lamp into it and the hum did not get louder.</P>


    4. The transformer on my amp power supply does acoustically buzz (loose plates??). The speaker lines do plug directly into the amp power supply. Could this make a difference? FYI the amp power supply supplies +34 and -34 to the amp boards and also acts as a speaker output box.</P>


    5. If I disconnect the line inputs into the 2 solid state amplifier boards (it is a 2-channel organ), the hum is reduced significantly. </P>


    6. In several areas of the audio processing circuitry (voicing, pre-amps, etc) I can induce a loud hum by passing my hand above certain components. Once I touch the component(s) this hum goes away. As long as I keep a finger on the component, I can pass my other hand over the same components without introducing any additional hum.</P>


    7. On the amp power supply, I noticed once that the transformer was buzzing louder than normal and that the hum out of the speakers was louder as well. I had also started getting some "rumbling" out of the amplifiers and one would intermittently be dead completely. I opened up the PS and discovered that the -34 lead on the filter caplooked "burned" and the solder joint was loose/cold. On the amp boards themselves, the 150-ohm resistors on the -34 power line were fried. I replaced the resistors in the amp boards and re-soldered the -34 supply line on the amp power supply. Then the hum got better. But,I don't know what would have caused this - could the solder joint have been bad all along, or could something have heated it up? Again, this is one of those 5000uF caps that replaced the original 1500uF caps.</P>


    8. Seemingly be design, the speaker leads run very close to several AC lines - there are AC lines for the leslie motor, the incoming power (connects to a twisted AC line), the amp power supply itself, etc. There does not appear to be andshielding in place. Could this be introducing hum into the speakers?</P>


    I don't know what to make of all this -maybe there are several problems going on. I thought I'd throw it all out there and see if you have any ideas. Thanks again for all your help.</P>


    -jim</P>
    <P mce_keep="true"></P>
    Jimmy Williams
    Hobbyist (organist/technician)
    Gulbransen Model D with Leslie 204

  • #2
    Re: Hum problems with Gulbransen Model D



    Lots of good info...</P>


    Speaker leads near power wiring should NOT be a problem.</P>


    You have a ground loop between your amp and the organ as evidenced by removing the input audio and hum reduces. Work this last...</P>


    Often the leads of large caps can come loose in the solder of circuit boards. Installing those larger caps MIGHT cause a problem related to inrush current at the start... that is a pretty big step up from 1500 to 5000... still, 1500 seems low...</P>


    The reason your rhythm box causes more hum than a lamp is EITHER it is emitting magnetic fields which couple into the amp OR the currents drawn due to rectification within the box have high peaks that the lamp does not. These high peaaks have high harmonic content and couple much easier. The wires to that switched outlet need to be twisted pair all the way... AND if the wires go anywhere near audio stuff, then fix it this way: Use the output of the plug to drive a pilot relay that controls the power you want. The relay will not generate peak currents and will be very low current.</P>


    For the transformer laminations: Get some transformer varnish and thin it down and flow into the laminations to help quiet them.</P>


    The buzz increasing on power down is normal... what happens is that the feedback paths in the amp go out of regulation as voltages collapse unevenly... so a bit of the ripple of the rectifier starts to come through.</P>


    The ground loop of the input audio is between the expression pedal and the amp obviously... the line probably isn't balanced but is single ended...</P>


    See if connecting just THE GROUND brings on the hum or whether it requires the audio lead itself to be connected... this is the next step for this problem...</P>


    This is enough for now... </P>

    Comment


    • #3
      Re: Hum problems with Gulbransen Model D



      Hi Fred.</P>


      Just checked - if I attach only the grounds from the audio linesto the amps, I do not get the hum. When I connect the "hot" side of the audio line, I do get the hum. Yes, these are unbalanced audio lines. The insulated cable contains another insulated lead (hot) and a shield (ground) from what I can tell.</P>


      In this organ the audio does not run through the expression pedal per-se; the expression pedal controls the brightness of a lamp (on the voicing/pre-amp board) which in turn changes the valuetwo light-dependent resistors (ORP-60) - one for each audio channel. So I guess then the ground loop is somewhere between the expression lamp and the amp. The weird thing is that to my ears at least the hum comes out of both sides, so it must be some part of the circuit that is shared - I guess they share a ground somewheredown theline but the audio signal itself is never mixed. I'll look at the schematics again.</P>


      About that switched AC outlet - it is a twisted pair, but very loose - probablygot all loosened up during the many times the unit was disconnected, transported, etc. I'll tighten up the twists but that is a separate problem from the ground loop, right? If so I'll just try to tackle the ground loop for now and address the other issues later.</P>


      Thanks,</P>


      jim</P>
      <P mce_keep="true"></P>
      <P mce_keep="true"></P>
      <P mce_keep="true"></P>
      Jimmy Williams
      Hobbyist (organist/technician)
      Gulbransen Model D with Leslie 204

      Comment


      • #4
        Re: Hum problems with Gulbransen Model D



        OK... NEXT step in troubleshooting... If you can borrow or find a BATTERY operated amp, see if the hum is coming out of the cables WITHOUT having the ground attached to anything else...</P>


        We are determining what is known as "common mode" noise here... and the cure is different depending on this test. It could also be done with differential scope probes, but it is unlikely you have those...</P>


        A pair of high impedance headphones (rare to find any these days) would also work... Radio Shack used to have a little battery amp in a small box that would work... If you know somebody with a battery guitar amp, that would work... Just need something that is ground isolated.</P>

        Comment


        • #5
          Re: Hum problems with Gulbransen Model D



          Fred,</P>


          I do have one of those old "Pignose" battery powered amps - just hope the battery terminals are OK (batteries were in there for a while before I got it...). I'll go dig it up.</P>


          So exactly what am I to do? Do I have to take both hot/ground sides of the audio line(s), disconnect them from the amp boards, and plug them into the pignose amp and listen for a hum?</P>


          Some other things that may or may not be impotrant:</P>


          1. The main power supply unit has two switched AC outlets (this is different than the external one I told you about) built into the chasis. The leslie motor plugs into one, and the separate amplifier power supply plugs into the other.</P>


          2. In the main power supply, there is a 600v, .0039uF ceramic cap across the 2 sides of the incoming AC line. The one side of both utility outlets is also in line with this cap. I don't know what would happen if this is open (it's obviously not shorted) - I don't have a capacitance meter.</P>


          3. The hum happens whether I use the receptacle on the amp chasis, OR if I plug the amp power supply directly into the wall (same outlet of course).</P>


          4. All the AC stuff in this organ is unpolarized, 2 prong. The organ is over 40 years old. I did try things like flipping the plug around but that didn't make any difference.</P>


          Thanks,</P>


          jim</P>
          <P mce_keep="true"></P>
          Jimmy Williams
          Hobbyist (organist/technician)
          Gulbransen Model D with Leslie 204

          Comment


          • #6
            Re: Hum problems with Gulbransen Model D



            Fred, </P>


            In addition to my last message - wanted to let you know that tonight I tightened up the 4 screws holding the laminates together on the buzzing transformer on the amplifier power supply. Believe it or not it made a significant difference - the transformer buzzes less and there is considerably less hum out of the speakers. I don't knowif there are any ramifications of tightening the screws but so far everything seems fine.I hope it was not just a "coincidence (who knows what other stray noise I can be picking up from lights, etc. The electric in this house still has very old, ungrounded outlets). So as of now I would say the hum is much "lower" in both pitch and volume but I'll keep an eye on it. Definitely a 60Hz hum coming from the main channel and as I said earlier if I touch a certain spot in the main audio circuit I can make the hum disappear (well almost - I know it will never be completely quiet). In fact, if I touch the keyboard chasis itself I can make that hum disappear too - tried that tonight - and it looks like several things are "grounded" to the keyboard chasis. I'll run the audio lines to the battery amp like you said and continue to follow your instructions. The tibia side just has that normal "hiss" like one would expect - not much of a hum at all now.</P>


            If it's humming louder again tomorrow I don't know what to say! </P>


            Thanks,</P>


            jim</P>
            Jimmy Williams
            Hobbyist (organist/technician)
            Gulbransen Model D with Leslie 204

            Comment


            • #7
              Re: Hum problems with Gulbransen Model D



              Tightening the bolts on the transformer is fine... Vibrations of the chassis can cause tubes to pick up microphonics... You can use the varnish too... lossen the bolts, varnish, then re-tighten.</P>


              It sounds like you need to install a grounded receptacle for your equipment.</P>


              IF YOU HAVE "Knob and Tube wiring" this will be a great source of hum that you will find hard to fight. The problem is actually the spacing of the two conductors which radiates a much larger magnetic field than modern day Romex type cables...</P>


              You would find it almost impossible to shield adequately against this... A lamp dimmer on K&amp;T wiring really radiates...!!!</P>

              Comment


              • #8
                Re: Hum problems with Gulbransen Model D



                Hi Fred.</P>


                Someof the knob and tube was upgradedto Romex - at a certain point in the basement. In other words, the old thick blacklines that come out of the breaker box go into a junction box and from that point they are wired to Romex. Some may still be hanging around. Some of the wiring to switches/outlets I saw does look like that old style cloth insulation.As per the inspector, mostreceptacles in the house are notgrounded - evenmost of the3-prong outlets. I do have two modern grounded 3-prong outlets on dedicated circuits that were originallyput in forwall air conditioners years ago by the previous owner.I tried running the organinto one of these outletsto rule out any problems with the house current itself - and I didn'tnotice anydifference - probably because the plug on the organ is unpolarized, 2-prong anyway? This house was built in 1893 and the electric was never completely re-done - just upgraded in dribs and drabs from what I can tell.</P>


                The organ has no tubes so I don't know what was "picking up" the hum from the transformer but at least that piece of the puzzle seems to be solved for now. I did start researching how to revarnish the laminations in case it comes to that. Never did that so I'm not quite sure how to - it seems that sometimes you have to "bake" the transformer to harden up the varnish?</P>


                About the AC outlet for the rhythm box - I did tighten up the twists and that helped noticably when the rhythm box is plugged in. Thanks for the tip. I'm keeping the rhythm box disconnected while I am doing all this other testing.</P>


                So the point I am at now is the remaining hum is definitely isolated to only the main audio channel. I don't quite understand the mechanics of why I can make the hum go away if I touch certain parts of that circuit. It kind of reminds me ofhow you can touch and hold a radio antenna to get better reception. </P>


                I'll try the battery amp test later today.</P>


                -jim</P>
                <P mce_keep="true"></P>
                Jimmy Williams
                Hobbyist (organist/technician)
                Gulbransen Model D with Leslie 204

                Comment


                • #9
                  Re: Hum problems with Gulbransen Model D



                  Yes, the "antenna" comment is exactly that... You pick up some hum signal from the wiring around you... WHEN you inject that into the circuitry AND it happens to be 180 degrees out of phase with the interfering hum, it can cancel the hum !</P>


                  You can flow thinned transformer varnish into the laminations... without baking it will take a lot longer to dry, but that is OK... It shouldn't take very much... you can probably get a little from a motor rewind shop.</P>


                  You should probably put a three wire cord on the organ... tie the gounding wire to chassis... and ahave a grounded plug to connect it to.</P>

                  Comment


                  • #10
                    Re: Hum problems with Gulbransen Model D



                    Hi Fredy.</P>


                    I hooked the problematic audio line to the battery amp - the hum is not there in the battery amp. Just the usual amp "hiss". Make of that what you will.</P>


                    If it is any help - I discovered something else (with the organ hooked up as normal). When you turn on the main channel stops, they turn on lamps that control LDRs (similar to how the expression pedal works). Turning these lamps on makes the hum get louder in the main channel - certain ones more than others. I'm not talking about actually playing any notes, just turning the stop tabs on. </P>


                    Another strange thing - when I hooked up the battery amp and actually played the notes, I was able to hear the tibia notes from the other channel softly "bleeding" into the main channel. Maybe a little cross-bleeding is normal - listening "up close" reveals a lot. </P>


                    And, something else - I recently hooked up a headphone jack using a 120-ohm 5W resistor to ground to cut the volume down(it is not hooked up now while doing all this troubleshooting) and listening up close through the phones I can definitely say the hum is isolated to the main channel. The headphones also revealed that the pedal notes - even with no stops on - bleed through into the signal - this bleeding signal sounds VERY distorted - like a buzzing kazoo - and of course soft. It is not a volume issue - when I turn the expression pedal up loud, and turn the pedal stops on, you can hear the bass notes - but you still getthe "pedalkazoo"under the surface. You can hear this through the speaker as well, but it is more "obvious" through the phones when the audio signal is attenuated but the "pedal kazoo"- which is NOT affected by the expression pedal - gets through anyway.It sounds like something is getting into the audio channel that shouldn't be there.</P>


                    The pedal pre-amp and voicing is all part of the main channel on this organ. Maybe I should start looking there??? Maybe disconnect the pedal pre-amps and see what happens? Or am I getting ahead of myself? Please advise. What is the next step now that we determined that the hum does not appear when the battery amp is used?</P>


                    Thanks,</P>


                    jim</P>
                    <P mce_keep="true"></P>
                    Jimmy Williams
                    Hobbyist (organist/technician)
                    Gulbransen Model D with Leslie 204

                    Comment


                    • #11
                      Re: Hum problems with Gulbransen Model D



                      Yes, not ahead of ourselves is a good plan, BUT keep ALL the symnptoms in mind.</P>


                      For the hum when the stop LDR's are turned on... Want to decide if the lamps are AC driven and the proximity to the LDR is coupling... IF YOU can, try shining a flashlight on the LDR to turn it on rather than the lamp... If the hum still is there, then the hum is upstream... if the flashlight method is quiet, then the lamps may be AC which is capacitively coupling to the LDR.</P>


                      On the kazoo thing...It is a possibility this is a quirk of the beast... This sounds like it MIGHT be digitizing noise... it could also be that if the pedals are gated by LDR for the pedal stops, the LDR may NOT cut off the sound completely. The organs OFTEN have a little stuff in the background that is FAR below the normal program level. This may also be the source of the cross bleeding.</P>


                      LDR's DO age and that may be part of the problem...and of course if light cross bleeds between them, then that is the fault of shielding of the light.</P>


                      There is obviously a ground loop between the main amp and the generator stuff... the single ended audio lines allow that. </P>


                      First thing is to connect a much heavier wire in parallel with the shield of the audio line and see what happens.</P>

                      Comment


                      • #12
                        Re: Hum problems with Gulbransen Model D



                        Hi Fred. I shined a flashlight on the LDR and this indeed did induce the hum. I'll connect that wire parallel to the audio shield and let you know what happens. Thanks for your valuable advice on how to methodically check these problems out!</P>


                        -jim</P>
                        Jimmy Williams
                        Hobbyist (organist/technician)
                        Gulbransen Model D with Leslie 204

                        Comment


                        • #13
                          Re: Hum problems with Gulbransen Model D



                          That means there is hum on on the generator side of the selection LDR's. </P>


                          There is a possibility that the hum level is the characteristic of the beast... It is hard for me to know just how loud the hum is... I would say it should be 60 Db below normal playing...</P>


                          The sources in the generator would be the power to them and what they pick up from the surrounding electromagnetic fields... could have weak filter caps allowing the regulators to dip slightly out of regulation... If transformer is close to this section, try putting a piece of steel between as a test to observe ANY change...</P>


                          You definetly want a three wire cord and grounded outlet to plug this into... even if it means retrofitting the organ and wall plugs.</P>

                          Comment

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